The Islamic State of Iraq and Syria (ISIS) militant group is facing a shortage of willing martyrs, after a recent string of defections following the loss of a key Syrian border town, activists within its territory say.

Since the loss of Kobane late last month to Kurdish forces, many militants marked for suicide operations within the group have fled or defected to rival militias, anti-ISIS activist group Raqqa Is Being Slaughtered Silently claims.

A source from inside the northern Syrian city of Raqqa, the de-facto ISIS capital, said that the militants had set up road blocks and imposed strict security checks to stop suicide bombers abandoning their tasks.

“There is great tension in Raqqa city, where ISIS suffered many defections in the past few days, said activist Abu Mohammed, according to UK newspaper the Daily Mail. “A source from within ISIS has confirmed that most of defections are from suicide bombers, where these defections are considered a painful blow to the group.”

Anyone caught attempting to dodge their deadly orders is executed, the activist said.

ISIS commanders reportedly select foreign militants for suicide missions because they often lack experience in fighting – with special training camps designed to filter out foreigners of little military to the group and prepare them for suicide operations, according to the Daily Mail.

ISIS is also under pressure from the heaviest U.S.-led air strikes since the start of the year.

At least 70 of its fighters have been killed by an escalation of the strikes since the group released a video showing it burning a captive Jordanian pilot to death last week, the Syrian Observatory for Human Rights said.

On Monday, ISIS reportedly withdrew some of its insurgents and equipment from areas northeast of the Syrian city of Aleppo, rebels and residents say, adding to signs of strain in the Syrian provinces of its self-declared caliphate.